Diffractive structure on inclined facets

ABSTRACT

A diffractive structure (100) has a substantially planar substrate (1). A set of facets (2) is formed in or on said substrate (1), the plane or planes in which the facets (2) lie being arranged at a non-zero angle to the plane of the substrate (1). Each facet (2) has a diffraction grating (5) formed thereon. The diffractive structure (100) will produce color over a wide range of viewing and illumination angles.

The present invention relates to a diffractive structure.

In many applications, it is desirable to have a diffractive structurehaving reflective properties which do not depend upon specific limitedillumination and viewing angles to produce colour.

For example, in security films, security holograms conventionallyconsist of a thermoformed plastic layer having a reflective aluminiumfilm deposited on the thermoformed layer. The hologram is formed bysurface relief. The absence of any absorption within the structure, andthe angular sensitivity of a holographic image, means that under diffuseillumination (such as dull daylight or rooms lit by many lamps), thehologram cannot be seen or can only be seen from within a very narrowrange of viewing angles.

As a further example, diffractive pigments, such as those disclosed inJP-A-63/172779, would benefit from the use of a diffractive structurewhich does not appear "washed out" and lacking in colour under normaloutdoor viewing conditions. JP-A-63/172779 discloses a pigment whichconsists of a multiplicity of particles each of which carries on itssurface grooves that form a diffraction grating. Since the gratings onthe particles suffer strong angular dependence and have no intrinsicabsorption, the diffractive colour effect will only be visible understrong, highly directional, illumination such as direct sunshine orspotlight illumination. Under diffuse illumination (for example on anovercast day), the pigment as disclosed in JP-A-63/172779 would appeargrey.

In EP-A-0303355, there is disclosed a hologram/diffractive medium havinga plurality of periodically-spaced stepped structures each of which isdistributed depthwise in the medium.

An object of the present invention is to provide a diffractive structurethat will produce colour over a wide range of viewing and illuminationangles.

According to a first aspect of the present invention, there is provideda diffractive structure, the structure comprising: a substantiallyplanar substrate; and, a set of facets formed in or on said substrate,the plane or planes in which the facets lie being arranged at a non-zeroangle to the plane of the substrate; the facets having a diffractiongrating formed thereon having a periodicity of 500 nm or less.

According to a second aspect of the present invention, there is provideda method of manufacturing a diffractive structure as described above,the method comprising the steps of: (A) producing a mould by machining asubstrate with repeated passes of a cutting tool, the tool cutting thesubstrate deeper on each pass of the tool thereby to produce a cut facehaving machining lines; (B) repeating step (A) to cut a further similarlined face opposite said first face thereby to produce a groove havingmachining lines on each opposed face; (C) producing a master from saidmould; and, (D) producing the diffractive structure from said master,each facet having formed thereon a diffraction grating corresponding tothe machining lines of the grooves in the mould.

According to a third aspect of the present invention, there is provideda method of manufacturing a diffractive structure as described above,the method comprising the steps of: (A) producing a mould by anisotropicetching in a silicon substrate to produce a plurality of facets on themould; (B) coating the mould with a resist layer; (C) writing the finestructure of the diffraction grating directly into the resist with anelectron beam or an ion beam; (D) producing a master from said mould;and, (E) producing the diffractive structure from said master.

Preferred features of the present invention are set out in the claimsbelow.

Thus, in an example of the invention, a prismatic surface structureconsisting of an array of substantially planar facets is formed in apolymer layer. These facets are typically in the region of 1 micron to100 microns across and are disposed at a predetermined angle to theplane of the polymer layer. A grooved surface, a ruled array oftetrahedra, square pyramids or a corner cube structure (in which thefacets are all squares) are examples of such a prismatic structure. Adiffraction structure is formed on the surface of each facet. Thissmaller structure may be (but is not restricted to) an array of grooves,a crossed grating or a 2-dimensional array of pits and peaks such as theknown "motheye" structure. The smaller structure will typically havedimensions ranging from half the facet size down to 0.1 micron. Thisstructure is preferentially metallised such that it is absorbing at someangles of incidence but produces strong diffraction at other angles.

The invention provides a diffractive structure that will produce colourover a wide range of viewing and illumination angles. The diffractivestructure can be manufactured in a simple manner using conventionalfilm-forming plastics.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a schematic perspective view of an example of a diffractivestructure according to the present invention;

FIG. 2 is a schematic perspective view of another example of adiffractive structure according to the present invention;

FIG. 3 is a schematic perspective view of a further example of adiffractive structure according to the present invention;

FIG. 4 is a schematic cross-sectional view through a polymer-filleddiffractive structure;

FIG. 5 is a drawing showing a representation of a light ray incident onthe diffractive structure;

FIG. 6 shows the distribution of light rays incident on the surface of apolymer layer of the diffractive structure;

FIG. 7 shows the distribution of light rays incident within the polymerlayer of the diffractive structure;

FIG. 8 shows the distribution of light rays incident on the grating ofthe diffractive structure;

FIG. 9 shows the distribution of light rays diffracted from the gratingof the diffractive structure;

FIG. 10 shows the distribution of light rays leaving the diffractivestructure; and,

FIG. 11 is a CIE (Commission International de l'Eclairage) colour chartshowing how the perceived colour varies with viewing angle.

In FIG. 1, there is shown a diffractive structure 100 formed by asubstrate 1 having an array of facets 2. The facets 2 in this exampleare provided by the triangular faces 2 of an array of square-basepyramids 3. The pyramids 3 are formed in or on the substrate 1 withtheir square bases 4 in the same plane so as to provide respective rowsof coplanar facets 2. The length of each side of the square bases 4 ofthe pyramids 3 may be in the range 1 μm to 100 μm. Each facet 2 of thepyramids 3 has upon it a diffraction structure such as a ruleddiffraction grating 5 formed by grooves or lines 6 having a regularspacing therebetween. The diffraction grating 5 in this example has aperiod of 300 nm (i.e. the spacing between successive lines 6 is 300 nm)with a height (i.e. the depth of the lines 6) of about 100 nm.

Instead of square-base pyramids 3, the two-dimensional facets 2 can beformed as the faces of triangular-base pyramids, regular tetrahedra (seeFIG. 2), a corner cube structure (in which all or substantially all ofthe facets are squares), or any other polyhedra or structures whichprovide an array of identical or substantially similar facets whichproject out of the plane of the substrate 1 at an angle between 0° and90° to the plane of the substrate 1.

Instead of a lined diffraction grating as described above, a2-dimensional array of pits and peaks such as the known "motheye"structure shown in FIG. 3 may be used as the diffraction grating 5.

Generally speaking, the length of the bases of the facets 2 may be inthe range 1 μm to 100 μm. The pitch of the diffraction grating 5 formedon a facet 2 may be in the range of 0.1 μm up to about half the facetsize, subject to a maximum of 0.5 μm.

Because the diffraction grating structure 5 is at an angle to the normalto the substrate 1, sub-wavelength diffraction gratings can be used,thus providing minimum dispersion (i.e. as little colour variation withangle as possible) and cut off of some colours which cannot be reflectedbecause their wavelength is too long.

Furthermore, opposing facets 2 allow short wavelength colours to betrapped because they are diffracted at a higher angle with respect tothe facets than the longer wavelengths.

Thus, in the preferred embodiment, there are two mechanisms for colourselection. Dazzling colour effects can be produced even under diffuselighting and without the use of pigments or dyes.

Two-dimensional facet structures as described, such as the faces ofarrays of pyramids or tetrahedra, reduce the sensitivity of thediffractive structure 100 to rotation of the structure 100 both in itsown plane and out of its plane that would be seen in a one-dimensionalfacet structure. Thus, as will be explained further below, thediffractive structure 100 of the present invention will produce colourimages under a wide range of lighting conditions and viewing angles.

Using a V-groove (i.e. one-dimensional) structure for the facets 2 meansthat the colour effect will depend on the angle of rotation of thesubstrate in its plane though relative insensitivity of the structure torotation out of its plane is still retained.

An example of a manufacturing process for producing the diffractivestructure 100 of the present invention will now be described.

First, a non-ferrous material such as brass or copper is machined usinga very sharp diamond tool (not shown) to form a mould which issubstantially identical to the diffractive structure 100 to be finallyproduced. The diamond tip of the tool may have an included angle of 30°.The tool is used to cut a groove to a first depth to provide a cut facehaving a length equal to the pitch of the diffraction grating to beformed. The tool is then used to cut the groove to a second depth to cutthe face to a length which is twice the pitch of the diffractiongrating. This process is repeated until the groove has been cut to thedesired depth, the tool being moved deeper into the mould material by adistance such that the face which is cut is cut by a length which isequal to the pitch of the diffraction grating to be formed on eachsuccessive pass of the tool. As a result of these successively deeperpasses of the machining tool, the structure of the diffraction gratingcomposed of the lines is formed from natural machining marks formedduring the successive passes of the tool. The groove thus cut willprovide a first row of facets in the material. The opposite row offacets 2' and other rows of facets, both parallel and orthogonal to thefirst row of facets, are then produced by machining further grooves in asimilar manner, the further grooves being parallel and perpendicular tothe first groove. The included angle of opposed facets may be 90° forexample, though this will depend on the geometry and height of thepyramids 3, tetrahedra or other polyhedra which provide the facets 2.

For speed of manufacture, a series of similar cutting tools can be usedto cut parallel rows of grooves to provide facets in a gang fashion.Orthogonal rows can be cut by moving the same or another gang of cuttingtools perpendicularly to the first row of facets.

An alternative technique for forming the mould is as follows. The mouldhaving the array 3 of square-base pyramids, triangular-base pyramids,regular tetrahedra, corner cube structure, or other polyhedra orstructure, is formed by anisotropic etching in silicon. This providesvery flat faces to the facets 2. The mould is then coated with a resistlayer. The fine structure which makes up the diffraction grating 5 isthen directly written into the resist with an electron beam or an ionbeam.

Whichever way the mould is formed, the mould is then electroformed toform a hard master, which is a negative of the mould and therefore alsoa negative of the diffractive structure 100 to be produced. The materialof the master needs to be hard enough to allow embossing of a plasticsmaterial or other material from which the diffractive structure 100 isformed. The master may be nickel or copper for example.

The master is then thermoformed to produce a negative replica of themaster in a polymer, in the same way as a conventional commercialhologram. Suitable polymers include polymethyl methacrylate orpolycarbonate. The facets 2 on the replica are then metallised with athin layer of a metal such as chrome, copper, nickel or aluminium toproduce the diffractive structure 100 of FIG. 1. The metallised layermay be 10 to 50 nm thick and is preferably discontinuous over the smallscale relief that forms the diffraction grating 5 so that thediffraction grating 5 is partially absorbing or transmitting, and isonly weakly specularly reflective.

The structure 100 is preferably then filled with a layer of material 7.The material of the layer 7 is transparent and may be a solvent-dryingor chemically-curing polymer, such that the structure 100 hassubstantially flat and parallel outer surfaces, and the internalstructure relief is filled with polymer 7, as shown in FIG. 2. Thepolymer layer 7 may conveniently be the adhesive which is used to fixthe diffractive structure 1 to a substrate on which it is mounted.

Light entering the polymer layer 7 is diffracted, absorbed, or reflectedby the facets 2. Undiffracted light is either absorbed by thediffraction grating 5 or is specularly reflected. If it is specularlyreflected, it passes to the neighbouring facet 2 where again it isabsorbed or specularly reflected. If the diffraction grating 5 isdesigned such that only 10% of light falling upon it can be specularlyreflected, only 1% may re-emerge after two such reflections. In theabsence of diffraction, the whole structure 100 is thereforesubstantially non-reflective and appears to the viewer to be black. Thediffraction grating 5 can be designed to reduce the amount of lightwhich is specularly reflected by ensuring that the pitch of the gratingis less than the wavelength of light incident on the grating and havingthe depth of the grating 5 such that the back reflection is cancelled byinterference. If the surface of the grating 5 is coated as suggestedabove with a "lossy" metal (i.e. one with a low reflectivity such ascopper, nickel or aluminium) or the metal is discontinuous across thelines 6 of the grating 5, then the incident light is absorbed ratherthan reflected. Diffraction will occur when the wavelength of incidentlight, the angle of incidence of the light, and the period of thediffraction grating 5 have the following relationship:

    λ/η=d.sin(φ)+d.sin(θ)

where λ is the wavelength of the light, η is the refractive index of thepolymer 7 filling the diffraction structure, φ and θ are the angles ofincidence and diffraction, relative to the normal 8 to the facet 2,respectively, and d is the period of the diffraction grating 5, as shownin FIG. 5.

The perceived colour of the structure 100 can be calculated by tracingthe paths of the rays which enter and leave the structure 100 bydiffraction. FIGS. 6 to 10 show how the distribution of rays changes asthe rays are first refracted at the polymer surface as they enter thepolymer, then diffracted at the facet, and refracted again as the raysleave the polymer.

FIG. 6 shows a polar plot of the intensity incident on a surface underdiffuse illumination, the surface in this case being the outer surfaceof the polymer layer 7. The intensity drops off with the cosine of theangle of incidence. This dependence is known as Lambertian illumination.

Because of refraction at the air-polymer boundary 9, the range of anglesover which the light rays propagate within the polymer layer 7 isreduced, as shown in the polar plot of FIG. 7.

As shown in FIG. 8, the rays then strike the diffractive grating 5 onthe surface of the facets 2 at a range of angles and a proportion of therays are diffracted. For the sake of reducing the complexity of thisdescription, it is assumed that (i) where diffraction is possible, allof the light is diffracted, and (ii) where diffraction is not possible,the light is absorbed or transmitted by the diffractive grating 5 asdescribed above. It is to be understood, however, that in practice thediffraction efficiency will vary with wavelength and angle.

Some of the light that is diffracted by the diffraction grating 5 on aparticular facet 2 is shadowed by the neighbouring facet 2 and does notleave the diffractive structure 100. The polar plot of FIG. 9 shows thedistribution of the diffracted rays from one facet 2 of the diffractivestructure 100 for three different wavelengths. These wavelengthscorrespond to the peaks of the visual colour response. The solid linerepresents blue light, the dotted line represents green light and thedashed line represents red light.

As the rays leave the polymer layer 7, they are again refracted at theair-polymer boundary 9. FIG. 10 shows the distribution of rays thatleave the diffractive structure 100 having been diffracted by one facet2.

The light rays leaving the one facet 2 can be added to the rays from theneighbouring facet 2 to produce a plot on a standard CIE (CommissionInternational de l'Eclairage) colour chart showing how the perceivedcolour varies with viewing angle. The chart is reproduced in FIG. 11.Over an 80° range of viewing angle (plus or minus 40° from the normal tothe polymer layer surface 9), there is very little variation inperceived colour. In this case, the diffractive structure 100 produces ayellow colour under diffuse illumination. Different colours may beproduced by altering the facet angles (i.e. the angle of a facet 2 tothe polymer layer surface 9) and the period d of the diffraction grating5.

Thus, the present invention provides a colour diffractive structure 100which retains a saturated colour when viewed in diffuse lighting over awide range of viewing angles.

The structure 100 can be mastered by conventional ruling techniques overa large area, and can be formed in a continuous polymer film by a singlestep embossing process similar to that used in the production ofholograms.

The colour primarily depends on the facet angles and the pitch of thediffraction grating, neither of which change significantly during wear.The diffractive structure 100 is therefore ideal for production of largevolumes of material. The diffractive structure 100 has particularapplication in security films on, for example, credit or debit cardswhere very many substantially identical diffractive structures 100 arerequired.

An embodiment of the present invention has been described withparticular reference to the examples illustrated. However, it will beappreciated that variations and modifications may be made to theexamples described within the scope of the present invention.

I claim:
 1. A diffractive structure, the structure comprising:asubstantially planar substrate; and, a set of facets formed in or onsaid substrate, the facets lying in at least one plane, the at least oneplane in which the facets lie being arranged at a non-zero angle to theplane of the substrate; the facets having a diffraction grating formedthereon; wherein the facets are provided by the faces of cubes.
 2. Adiffractive structure, the structure comprising:a substantially planarsubstrate; and, a set of facets formed in or on said substrate, thefacets lying in at least one plane, the at least one plane in which thefacets lie being arranged at a non-zero angle to the plane of thesubstrate; the facets having a diffraction grating formed thereon;wherein the facets are provided by a corner cube structure.
 3. Adiffractive structure, the structure comprising:a substantially planarsubstrate; and, a set of facets formed in or on said substrate, thefacets lying in at least one plane, the at least one plane in which thefacets lie being arranged at a non-zero angle to the plane of thesubstrate; the facets having a diffraction grating formed thereon;wherein the diffraction grating is provided by a crossed grating on thefacets.
 4. A diffractive structure, the structure comprising:asubstantially planar substrate; and, a set of facets formed in or onsaid substrate, the facets lying in at least one plane, the at least oneplane in which the facets lie being arranged at a non-zero angle to theplane of the substrate; the facets having a diffraction grating formedthereon; wherein the facets are formed of a polymeric material coatedwith a layer of metal.
 5. A structure according to claim 4, wherein themetal layer is discontinuous over the diffraction grating on the facets.6. A method of manufacturing a diffractive structure the structurecomprising: a substantially planar substrate; and, a set of facetsformed in or on said substrate, the facets lying in at least one plane,the at least one plane in which the facets lie being arranged at anon-zero angle to the plane of the substrate; the facets having adiffraction grating formed thereon, the method comprising the stepsof:(A) producing a mould by machining a substrate with repeated passesof a cutting tool, the tool cutting the substrate deeper on each pass ofthe tool thereby to produce a cut face having machining lines; (B)repeating step (A) to cut a further similar lined face opposite saidfirst face thereby to produce a groove having machining lines on eachopposed face; (C) producing a master from said mould; and (D) producingthe diffractive structure from said master, each facet having formedthereon a diffraction grating corresponding to the machining lines ofthe grooves in the mould.
 7. A method according to claim 6, whereinfurther lined grooves parallel to the first lined groove are cut byrepeating steps (A) and (B).
 8. A method according to claim 6, whereinfurther lined grooves orthogonal to the first lined groove are cut byrepeating steps (A) and (B).
 9. A method according to claim 6, whereinfurther lined grooves parallel to the first lined groove aresimultaneously cut with the first lined groove by a ganged series ofcutting tools.
 10. A method according to claim 6, wherein further linedgrooves orthogonal to the first lined groove are simultaneously cut by aganged series of cutting tools.
 11. A method according to claim 6,wherein the master is produced by electroforming the mould.
 12. A methodaccording to claim 6, wherein the diffractive structure is produced bythermoforming the master.
 13. A method according to claim 12, furthercomprising the step of coating the facets of the diffractive structurewith a metal layer.
 14. A method according to claim 13, wherein themetal layer is discontinuous over the diffraction grating on the facets.15. A method of manufacturing a diffractive structure, the structurecomprising: a substantially planar substrate; and, a set of facetsformed in or on said substrate, the facets lying in at least one plane,the at least one plane in which the facets lie being arranged at anon-zero angle to the plane of the substrate; the facets having adiffraction grating formed thereon, the method comprising the stepsof:(A) producing a mould by anisotropic etching in a silicon substrateto produce a plurality of facets on the mould; (B) coating the mouldwith a resist layer; (C) writing the fine structure of the diffractiongrating directly into the resist with at least one of an electron beamand an ion beam; (D) producing a master from said mould; and, (E)producing the diffractive structure from said master.